Tapped-transformer system



May 29, 1956 T. HEHENKAMP TAPPED-TRANSFORMER SYSTEM Filed June 29, 1953 INVENTOR T HEODORUS HE HE N KAMP nited States Patent TAPPED-TRANSFORMER SYSTEM Theodorus Hehenkamp, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, C0nn., as trustee Application June 29, 1953, Serial No. 364,746 Claims priority, application Netherlands July 31, 1952 2 Claims. (Cl. 32343.5)

This invention concerns tapped-transformer systems. More particularly, the present invention concerns tappedtransformer systems suitable for controlling airport lighting systems.

Devices have been suggested for use in airport or area lighting systems, comprising a potentiometer adapted to be connected to a voltage supply and having several tappings for the obtaining of different voltages. The tappings are controlled each by a separate switch, each switch is operated by means of an electromagnetic relay and can be connected to the load, the energizing winding of each relay being adapted to be connected to an alternating voltage supply via a separate operating switch.

If the load is an airport lighting system it is thus possible to produce a more or less intense lighting of the airport according to the weather conditions, such as fog, rain, bright weather and so forth. On one of the operating switches of the energizing winding being closed the corresponding relay is energized and the switch coupled thereto is closed with the result that the selected tapping of the potentiometer, which has a definite voltage, is connected to the lighting system.

In this case it is necessary that if another operating switch of another energizing winding is to be closed in order to supply a different voltage to the load, the first switch, which connects the selected tapping to the load, should be open since otherwise a short-circuit would be established through a portion of the potentiometer. In this case the fact that the operating switch of the first-mentioned energizing winding is open before the closure of another operating switch does not provide a suflicient guarantee against the said short-circuit, since the combination of armature and switch which is operated by the relay, which, by now, is no longer energized, can continue to stick so that the load remains connected to the corresponding tapping.

In accordance with the present invention this difficulty is mitigated in that a common supply lead to the alternating voltage supply of a device of the type described includes the series combination of an inductance and a capacitor which are proportioned so that the series combination is at least substantially in resonance at the frequency of the alternating voltage. In addition, each electromagnetic relay comprises an auxiliary contact which in the energized condition of the relay short-circuits said capacitor, in some cases via a resistor.

The present invention is based on the fact that the impedance of the energizing Winding of a relay in the non-energized condition, that is, with open armature, is generally considerably lower, for example times lower, than the impedance of the winding of a relay in the energized condition with attracted armature.

The term substantially resonant is to be understood to mean essentially that the series combination absorbs such portion of the alternating voltage that the residual voltage at the corresponding energizing winding suflices to energize the relay, whereas after the capacitor is shortcircuited the inductance absorbs such portion of the ing with open armature of the relay is insufficient to energize the relay.

According to another feature of the invention the impedance of the said inductance is at least substantially equal to that of each of the energizing windings when the relay is not energized.

In order that the invention may be clearly understood it will now be described with reference to the accompanying diagrammatic drawing, given by way of example.

In the figure, an alternating voltage supply 1 is connected to a transformer 2, the secondary 3 of which comprises tappings 4, 5 and 6, thus acting as a potentiometer. The tappings 4, 5 and 6 may be connected to the load, which may be, for example, an airport lighting system 10, as desired via switches 7, 8 and 9.

The switches 7, 8 and 9 are coupled to the armatures 11, 12 and 13 of electromagnetic relays 14, 15 and 16, the energizing windings 17, 18 and 19 of which may be connected to a terminal of the alternating voltage supply 1 via operating switches 20, 21 and 22 respectively.

In accordance with the present invention, the other ends of the energizing windings 17, 18 and 19 are connected to the other terminal of the alternating voltage supply 1 via the series combination of an inductance 23 and a capacitor 24. In addition, the relays comprise auxiliary contacts 25, 26 and 27 respectively. When a particular relay, such as, for example, the relay 14, is energized, the auxiliary contact associated with the energized relay, such as 25, short-circuits the capacitor 24. The inductance 23 and the capacitor 24 are proportioned so that their series combination is more or less in resonance with the frequency of the alternating current supply 1.

In the position shown in the figure the switch 20 is closed, the energizing winding 17 still being supplied via the series combination of the inductance 23 and the capacitor 24, since prior to energization of the relay 14, that is, with the armature 11 released, the auxiliary contact 25 is still open. Since the series combination is in resonance, the energizing winding 17 has supplied to it the maximum alternating voltage but for the ohmic voltage drop across the series combination, and relay 14 is energized. The switch 7 and the auxiliary contact 25 are thus closed, as shown in the figure, and the load 10 is connected to the lowest voltage, while at the same time the capacitor 24 is short-circuited.

The impedance of each of the energizing windings 17, 18 and 19 is assumed to be, for example, about ohms with open armature and about 1000 ohms with closed armature. Thus, if the impedance of the inductance 23 is, for example, also 100 ohms, the alternating voltage at the energizing winding 17 subsequent to the capacitor 24 being short-circuited is about 1000/ 1100:10/ 11 part of the maximum supply voltage.

Upon the switch 21 then being closed, the energizing winding has a voltage supplied to it via the inductance 23. Since the armature 12 is still open the winding 18 has an impedance of 100 ohms similarly to the inductance 23 so that the voltage of the winding 18 is only half the supply voltage, with the result that the relay 15 can not be energized. The device is thus protected from short-circuit of the portion of the secondary 3 comprised between the tappings 4 and 5.

Even if the switch 20 is open and the armature 11 continues to stick the device is free from short-circuit.

In addition, it is to be noted that the voltage source to which the potentiometer 3 is connected for the obtaining of the diflterent voltages at the load 10 may alternatively be a direct voltage source.

The device of the present invention may be used for suitable loads other than airport lighting systems, but is particularly suitable for the latter by reason of the high requirements as to dependability and safety for the airport and because of the comparatively high power and large distances for which the use of relays, generally operated from a distance, is necessary.

It is to be understood that the invention is not limited to the details disclosed but includes all such variations and modifications as fall within the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. A device for selectively applying one of a plurality of predetermined voltages to a given load comprising a tapped transformer connected to a voltage source, said transformer having a secondary winding having a plurality of taps for providing said plurality of voltages, a. switch connected between each of said taps and said load, an electromagnetic-relay operatively coupled to each of said switches, a plurality of operating switches, each of said relays having an energizing winding connected to one of said operating switches, and a common lead including a series combination of an inductor and a capacitor connecting said relays via said operating switches to a source of alternating voltage, said inductor and said capacitor being tuned substantially to series resonance at the frequency of the voltage of said alternating voltage source, each of said electromagnetic relays having an auxiliary contact and means connecting said auxiliary contact to short-circuit said capacitor in the energized condition of the respective relay.

2. A device, as set forth in claim 1, wherein the impedance of said inductor is substantially equal to that of each of the energizing windings in the non-energized condition of the respective relay.

References Cited in the file of this patent UNITED STATES PATENTS 

